在实体之外无法知道系统的输入和输出边界。
Outside entities cannot know where the input and output boundaries of your system are.
例如，用例的前置及后置条件、输入和输出定义服务依赖关系、边界和数据要求。
For example, the pre - and post-conditions, inputs, and outputs of the use case define the service dependencies, boundary, and data requirements.
边界类处理软件系统的输入与输出。
Boundary classes handle the input and output functions required by a software system.
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